THE EYE AS AN OPTICAL INSTRUMENT. 195 



In fact, although the crystalline lens looks so beautifully 

 clear when taken out of the eye of an animal just killed, it is 

 far from optically uniform in structure. It is possible to see 

 the shadows and dark spots within the eye (the so-called ' en- 

 toptic objects ') by looking at an extensive bright surface the 

 clear sky, for instance through a very narrow opening. And 

 these shadows are chiefly due to the fibres and spots in the lens. 



There are also a number of minute fibres, corpuscles and 

 folds of membrane, which float in the vitreous humour, and are 

 seen when they come close in front of the retina, even under 

 the ordinary conditions of vision. They are then called muscce 

 volitantes, because when the observer tries to look 1 at them, 

 they naturally move with the movement of the eye. They seem 

 continually to flit away from the point of vision, and thus look 

 like flying insects. These objects are present in every one's eyes, 

 and usually float in the highest part of the globe of the eye, out 

 of the field of vision, whence on any sudden movement of the 

 eye they are dislodged and swim freely in the vitreous humour. 

 They may occasionally pass in front of the central pit, and so 

 impair sight. It is a remarkable proof of the way in which we 

 observe, or fail to observe, the impressions made on our senses, 

 that these musace volitantes often appear something quite new 

 and disquieting to persons whose sight is beginning to suffer 

 from any cause ; although, of course, there must have been the 

 same conditions long before. 



A knowledge of the way in which the eye is developed in man 

 and other vertebrates explains these irregularities in the struc- 

 ture of the lens and the vitreous body. Both are produced by 



becoming for a time faintly luminous as long as they receive violet and blue 

 light. The bluish tint of a solution of quinine, and the green colour of 

 uranium glass, depend on this property. The fluorescence of 'the cornea and 

 crystalline lens appears to depend upon the presence in their tissue of a very 

 small quantity of a substance like quinine. For the physiologist this property 

 is most valuable, for by its aid he can see the lens in a living eye by thruw- 

 ing on it a concentrated beam of blue light, and thus ascertain that it is placed 

 close behind the iris, not separated by a large ' posterior chamber,' as was long 

 supposed. But for seeing, the fluorescence of the cornea and lens is siuiply 

 disadvantageous. 



i Vide supra, p. 189. 



02 



